Electrophotography is the electrical recording of information in the form of a two-dimensional optical image. Electrophotographic processes, such as xerography, normally use materials consisting of a support coated with a photoconductive substance(s). Such processes operate by virtue of the fact that the photoconductive substance(s) is normally an electrical insulator in the dark (having a specific resistivity of about 10.sup.15 ohm-cm.) and becomes a semiconductor in the presence of light (having a specific resistivity of about 10.sup.11 ohm-cm.).
Thus, in electrophotographic processes, the photoconductive layer is charged either positively or negatively in the dark and light is then impinged on the charged surface through a master or an image of the master is projected onto the charged surface. In those areas where light strikes, the surface becomes electrically conductive and allows the surface charge to dissipate into the electroconductive support. Where no light strikes the charged surface, the electrostatic charge remains in the form of a latent image. This remaining image is then developed by powdering (or otherwise contacting) the surface with a pigment (toner) bearing the opposite charge. Both positive and negative images can be obtained by the appropriate choice of master and toner.
Inorganic compounds, such as seleniumm and zinc oxides, as well as various organic compounds, such as anthracene, carbazoles, poly(N-vinylcarbazole), etc., have been previously used as photoconductors in electrophotography. The organic materials have many advantages; e.g. they are in most instances soluble in conventional solvents and may be easily cast as films, etc.
An extensive technical and literature review of electrophotography and the various organic photoconductors useful therein is entitled "Organic Photoconductors in Electrophotography" by L. I. Grossweiner (published and sold by M/K Systems; Marblehead, Mass. (1970)).
One of the primary problems associated with organic photoconductors in electrophotography is their low photosensitivity (i.e., their low rate of decay of electrical potential when illuminated). Many compounds have been used as "activators" in combination with such organic photoconductors to improve their photosensitivity, as illustrated by L. I. Grossweiner cited above. The effective activators generally discolored the photoconductor and thus rendered the system unsatisfactory in direct electrostatic copying processes and the like which require an essentially colorless system.